Add NEC models from drc

[beckyfisher]

The drc package includes an NEC function, that fits 2, 3 and 4 parameter NEC models. All 3 differ slightly from the original formulation of the exponential decay model in Fox 2010/Peres that is currently implemented.
The model formula is:
f (x) = c + (d − c) exp(−b(x − e)I(x − e)) + d2/(1 + exp(b2(log(x) − log(e2))))

With three variations:
NEC.2(upper = 1, fixed = c(NA, NA), names = c("b", "e"), ...)
NEC.3(fixed = c(NA, NA, NA), names = c("b", "d", "e"), ...)
NEC.4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), ...)

The parameter e in NEC in "drc" corresponds to the parameter c’ in Pires et al (2002), the parameter b in NEC in "drc" corresponds to the parameter m’ in Pires et al (2002), the parameter d in NEC in "drc" corresponds to the parameter l’ in Pires et al (2002), and finally the parameter c in NEC in "drc" (the lower horizontal limit) is (implictly) fixed at 0 in Pires et al (2002).
More info at: https://cran.r-project.org/web/packages/drc/drc.pdf